Prior to setting forth a short discussion of the related art, it may be helpful to set forth definitions of certain terms that will be used hereinafter.
The term “MIMO” as used herein, is defined as the use of multiple antennas at both the transmitter and receiver to improve communication performance (measured by predefined metrics known in the art). MIMO offers significant increases in data throughput and link range without additional bandwidth or increased transmit power. It achieves this goal by spreading the transmit power over the antennas to achieve spatial multiplexing that improves the spectral efficiency (more bits per second per Hz of bandwidth) or to achieve a diversity gain that improves the link reliability (reduced fading), or increased antenna directivity. Several MIMO receiving schemes are known to be used such as channel estimation and blind scan.
The term “beamforming” sometimes referred to as “spatial filtering” as used herein, is a signal processing technique used in antenna arrays for directional signal transmission or reception. This is achieved by combining elements in the array in such a way that signals at particular angles experience constructive interference while others experience destructive interference. Beamforming can be used at both the transmitting and receiving ends in order to achieve spatial selectivity.
The term “beamformer” as used herein refers to RF circuitry that implements beamforming and usually includes a combiner and may further include switches, controllable phase shifters, and in some cases amplifiers and/or attenuators. For the sake of simplicity, in the following text, any referral to “phase” may also include a “gain” or “attenuation” of the signal.
The term “look through” method as used herein refers to receivers that use a single receive antenna in each beamformer at a time in order to carry out channel estimation and obtain the channel information (components of H being the channel matrix).
The term “Receiving Radio Distribution Network” or “Rx RDN” or simply “RDN” as used herein is defined as a group of beamformers as set forth above.
The term “hybrid MIMO RDN” as used herein is defined as a MIMO system that employs two or more antennas per channel (N is the number of channels and M is the total number of antennae and M>N). This architecture employs a beamformer for each channel so that two or more antennas are combined for each radio circuit that is connected to each one of the channels.
In hybrid MIMO RDN receiving systems, when the phases of the received signals from each antenna are properly adjusted or tuned with respect to one another, the individual signals may be combined and result in an improved performance of the receiving system.
FIG. 1 shows a non-limiting example of a standard 2×2 MIMO radio 20 with two antennas A and B communicating with a base station 10 having two transmit antennas radiating Tx1 and Tx2 according to the prior art. For the rank 2 (i.e., MIMO rank>1 in general), each transmit antenna will transmit portion of the two data streams with pre-coding weight W. While each receive antenna A and B receives both data streams, the baseband processes channel estimation separates them using the knowledge of pre-coding weight. It is noted that the receiver always knows the transmit pre-coding weight W, either by its own feedback to transmitter or by being informed by the transmitter in advance. The pre-coding weight W is configured to de-correlate the data streams by forming two orthogonal beams: one for each data stream, at the receiver.